placenta development trait embryonic cell/tissue morphology trait trophectoderm morphology trait VT:0005028 Trait.ontology Any measurable or observable characteristic related to the shape, structure, color, or pattern of the outermost layer of cells in the blastodermic vesicle, which will develop into the trophoblast layer and then contact the endometrium and take part in establishing the mammalian embryo's means of nutrition.